The following correction was printed in the Observer's For the record column, Sunday May 15 2005

"The article below muddles food manufacturers when it alleges that the Prince of Wales's Duchy Originals oaten biscuits 'have almost as much in common with the heir to the throne as they do with TV football presenter Gary Lineker. Although ingredients are grown at Highgrove, they are baked by Walkers food company, for which Lineker advertises crisps'. They are actually baked by Walkers Biscuits, a family business in Aberlour on Speyside, which has no connection to Walkers Crisps or Gary Lineker."

For centuries, goods bearing a royal seal of approval have enjoyed a considerable cachet. Even in Britain's cynical consumer society, it is still an imprimatur for which people are prepared to pay - an extra 300 per cent, it seems.

Bottled water with Prince Charles's organic label is on sale in supermarkets at four times the price of water that cannot boast the Duchy Originals brand, even though both originate from the same Highland well.

Sporting a stylish bottle and embossed with the Prince of Wales's brand, Duchy Originals Royal Deeside Spring Water was on sale in Sainsbury's last week at 99p for 500ml.

But the same chain offers Deeside natural mineral water taken from the same Pannanich wells in Aberdeenshire - admittedly in more modest packaging - at 99p for two litres.

Martin Simpson, managing director of the Deeside Water Company, which makes both brands, admitted the two were 'almost identical'.

At the Ritz Hotel in central London, whose glamour has drawn many royal visitors, the cost of Prince Charles's water is steeper still. Patrons preferring to quench their thirst from the brass taps in the bathrooms could acquire more than 80,000 litres for the price - £5.50 - of 750ml of the royal-sanctioned beverage.

The Pannanich wells, a short walk from the Queen's Balmoral estate, are famous for their healing properties.

Simpson added that, in keeping with industry regulations, the Duchy Originals water came from a different Pannanich source from its cheaper rival and there were minute differences in composition. A duchy spokesman said all profits went to the Prince of Wales Charitable Foundation.

The revelation has led to increased scrutiny of the Duchy Originals brand which last year made profits of more than £1 million. The brand's Oaten biscuits have almost as much in common with the heir to the throne as they do with TV football presenter Gary Lineker. Although ingredients are grown at Highgrove, they are baked by Walkers food company, for which Lineker advertises crisps.

It is not the first time a major brand has been in a row over the cost of bottled water. Last year Coca-Cola was criticised when its bottled water went on sale at 95p despite coming from the tap.